This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The objective of this study are to identify 1) the role of regulatory T cells in SIV pathogenesis, 2) In vivo expansion of regulatory T cells and its effect on SIV pathogenesis, and 3) Age-dependent differences in Treg frequencies and function.